1. Field of the Invention
The present invention relates to an image-forming method in a laser color printer, colored image copying machine, or the like and, more particularly, to a method of forming images by superimposing a plurality of toner images on an image retainer.
2. Description of the Prior Art
Copying machines have been known in which a colored original picture is illuminated with light, and the resultant light is decomposed into several components by color filters. The light components are then caused to fall on an electrically charged image retainer to form an electrostatic image, which is then developed by applying a powdered toner of a color, such as a yellow pigment, magenta, Cyan, or a black pigment, which corresponds to one of the color filters. The obtained toner image is transferred to paper. Subsequently, a second electrostatic image is formed in the same manner, and this image is then developed by applying a powdered toner of another color. The resultant toner image is transferred to the same paper. Similar operations are performed to superimpose toner images on the paper. In this kind of copier, each time one development in one color is completed, the toner image is transferred to the paper, thus necessitating a mechanism for rotating or reciprocating the paper. This makes the machine bulky. Also, the time necessary to form the whole image is lengthened. Another problem is that it is difficult to make coincident positions at which toner images are transferred to the paper.
In an attempt to overcome the foregoing problems with the aforementioned copier, an electrophotographic printer has been proposed in Japanese Patent Laid-open No. 144452/1981, where toner images are superimposed on an image retainer and the superimposed images are subsequently transferred to paper at once. In this apparatus, in order to prevent toner images previously formed on the retainer from being disturbed by later developments, the powdered toner is caused to make a flight from developer-feeding carriers under an oscillating electric field during the second and subsequent developments. Thus, the toner adheres to the retainer, and this process is called noncontact development. Although the toner is caused to fly under the control of the oscillating field in a noncontact manner as described above for permit superimposition of toner images on the retainer, it is still likely that the powdered toner previously stuck to the retainer is transferred back to the developer-feeding carriers or the previously formed toner images are disturbed by the action of the electric field. This makes it impossible to vividly reproduce colored images. The phase of the AC component of the oscillating electric field produces an electric force that acts on the toner existing between the retainer and each carrier in the direction to move it from the carrier to the retainer, plus a second electric force acting on the toner in the opposite direction. The aforementioned undesired phenomenon is explained by this second force. If the DC component of the oscillating field is adjusted to prevent the toner from being transferred back, fog will immediately be produced. Hence, it is quite difficult to control the travel of toner particles by the oscillating field in such a way that neither back-transfer nor fog takes place.